1. Field of the Invention:
The present invention is directed generally to a failed electrical component detector used in conjunction with an electric machine having a rotating member, and more particularly to a failed fuse detector used in conjunction with a brushless exciter.
2. Description of the Prior Art:
There are many kinds of multiphase electric machines having rotating members which carry electrical components, the failure of which must be detected. One example of such an electric machine is the brushless exciter. The basic function of a brushless exciter is to generate an alternating current and convert that current into a direct current that is used to produce the rotating field for an electrical generator. Rectification is accomplished through the use of multiple diode circuits which are rigidly mounted to the brushless exciter's rotor. For a six-phase wye system a minimum of six diodes is required. Typically, however, many more are used due to the necessity of providing spare legs, multiple pole designs and very high current requirements. Where high current capabilities are needed, many diodes are connected in parallel to reduce the current flow through each individual diode.
To protect such diodes from degradation due to overcurrent application, and to protect the exciter's operation from the effects of a shorted diode, designs typically include fuses series connected with the diodes. In applications that utilize many diode-fuse combinations in parallel, the failure of a single fuse may not have an adverse effect on the remaining diode-fuse combinations. Although each of the remaining parallel fuses will be required to carry an increased current load, this current is usually within the design capability of the fuse. However, a single fuse failure, while not adversely effecting the performance of the brushless exciter, cannot be ignored for long periods of time since the other fuses are working under a slight overcurrent condition and any subsequent fuse failure could have serious effects on the brushless exciter's ability to properly function. Thus, detection of fuse failure is important to enable the operator to shut the exciter down in an orderly fashion and replace the failed fuses. Furthermore, detection of fuse failure is critical if an entire phase of fuses fails thus requiring an immediate emergency shutdown of the system. For those reasons, it is important to be able to determine not only if any fuses have failed, but how many have failed and the specific locations in the circuit of those failures. Because brushless exciter rotors typically rotate at either 1800 RPM or 3600 RPM, inspections by an operator have heretofore been impossible without the aid of specially designed equipment.
There have been several attempts to design equipment for indicating failed fuses. One approach is disclosed in U.S. Pat. No. 4,635,045 which is directed to a Failed Fuse Detector And Detecting Method For Rotating Electrical Equipment. A current transformer is disposed about each fuse in such a way that it uses the conductor which carries current to the fuse as the primary winding of the current transformer. A light emitting diode is connected in series with the secondary winding of the current transformer. The emitted light, indicating a current flow through the fuse, is sensed by a stationary photodetector. Electronic circuitry is used to synchronize the detection operation with the rotation of the brushless exciter so that each fuse is inspected at a time when it is expected to be carrying its maximum current. Because this method looks for the existence of current through each fuse, it operates in a failsafe manner.
Another approach is disclosed in U.S. Pat. No. 3,866,196 which utilizes a projectile that, upon a fuse failure, is allowed to extend radially from the surface of the exciter rotor and be observed through the use of a stroboscopic light. A retaining wire holds the projectile in place. That retaining wire is connected electrically in parallel with the fuse. When the fuse fails, the current has to pass through the retaining wire causing it to overheat, rupture, and release the projectile. The idea of using a current path parallel to the fuse to activate a visible signal upon a fuse failure is also disclosed in U.S. Pat. No. 3,030,531. In that patent, instead of rupturing a retaining wire, the parallel current is used to activate a lamp which can be observed with a stroboscopic light.
A third approach is exemplified by U.S. Pat. No 4,635,044. The disclosed invention operates on the principal that, if a fuse is conducting an electric current, a magnetic field will exist around it and will move with the fuse as the exciter rotor rotates. That magnetic field can be remotely sensed by various devices and the conducting status of the fuse can be determined. Because any specific diode and its associated fuse do not conduct current during the entire 360.degree. of the exciter rotor, means are provided to selectively activate fixed magnetic field sensors that are disposed about the rotor at locations where the various fuses are, by design, expected to have current flowing through them. Further, electronic circuitry allows for automatic display, alarm and shutdown sequences to be incorporated within the device.
In view of the intricacy associated with the prior art in failed fuse detectors for brushless exciters, the need exists for a failed fuse detector which requires minimal modification of the brushless exciter, requires no synchronization or sophisticated electronics, is reliable, and which provides a visual indication to the operator without the use of any specialized equipment.